DE 10 2004 028 889 A1 shows a metering pump, comprising a sealing cylinder, a piston rod which can be moved axially in the sealing cylinder, an outlet valve housing which is arranged on the sealing cylinder on the outlet side, and an outlet valve body. At its outlet-side end, the sealing cylinder has an inwardly directed flange which has a radially flattened edge which faces the outlet valve housing and forms a valve seat for the valve body. The sealing cylinder forms and comprises a conveying space, a spring which is arranged in the conveying space being supported against the flange. The spring is supported at the other end on the piston rod and prestresses the piston rod counter to the conveying direction. On the inlet side, the cylinder element has axially running slots which form a fluidic connection from a pump space to the conveying space. The slots have a control edge which runs normally with respect to the movement axis of the piston rod and delimits the conveying space. The piston rod has a cylindrical projection on the outlet side, a circumferential sealing element being arranged on the projection. It is disadvantageous that the sealing element has high wear when the sealing element moves past the control edge and is in contact with the latter in the process. Furthermore, high pressure fluctuations are disadvantageously produced in the grooves, with the result that the piston rod experiences radially acting transverse forces and, as a result, high friction of the piston rod on the sealing cylinder is produced, as a result of which the service life of the pump is greatly restricted. Finally, the mass to be moved of the piston rod is great, since the diameter of the piston rod is adapted over its entire extent to the internal diameter of the sealing cylinder.
EP 1 878 920 A1 shows an electromagnetic metering pump having a coil which is wound on a coil former, a housing which supports the coil former, a core flange which is let into the housing and has an outlet channel, and a connection piece with an inlet channel, which connection piece is screwed into the housing opposite the core flange. The core flange has a hollow body which forms a sealing cylinder, has an approximately hollow-cylindrical shape, and penetrates the coil former approximately over the entire axial extent of the coil former. Furthermore, an actuator is provided comprising a ferromagnetic armature and a piston rod which is arranged in the ferromagnetic armature, the piston rod penetrating the hollow body of the core part, and the ferromagnetic armature being arranged in a pump space which is formed between the hollow body of the ferromagnetic core flange and the connection piece. A fluid channel which opens into a radial bore in the cylindrical section of the core part is provided between the hollow body of the ferromagnetic core flange and the coil former, it being possible for fluid which is to be conveyed from the pump space by a stroke movement of the armature to be pumped via the fluid channel and the radial bore into a conveying space which is arranged between the piston rod and the outlet channel. The fluid channel which surrounds the sealing cylinder is provided disadvantageously, which fluid channel prevents a compact overall design of the metering pump.